Infection and disease

Disease

Disease can be thought of as any condition that impairs the normal functions of an organism.

Diseases can be divided into two broad categories: Non-infectious diseases Infectious diseases

Non-infectious diseases:

Are not caused by an infectious agent

May be the result of more than one factor: environmental, genetic and biological factors may contribute to their development

Cannot be transmitted from one person to another

Non-infectious diseases include:

Nutritional (deficiency diseases) e.g. scurvy, anorexia

Genetic Sickle cell disease, cystic fibrosis, Huntington disease Congenital birth defects Chromosomal disorders/genetic changes leading to cancers

Environment (social diseases) Smoking - Cancer

Mental diseases Bipolar disorder, schizophrenia

Degenerative diseases Alzeihmers disease, Parkinsons disease

Diseases of physiological malfunction Cardiovascular disease

Autoimmune diseases Type I diabetes

Infectious diseases:

The germ theory of disease, also called the pathogenic theory of medicine, is a theory that proposes that micro-organisms are the cause of many diseases.

Highly controversial when first proposed Now a cornerstone of modern medicine and clinical

microbiology, leading to such important innovations such as antibiotics and hygienic practices.

Louis Pasteur, Robert Koch and Joseph Lister are given credit for it’s acceptance in the later half of the 19th century (around the time Warrnambool was settled).

Germ theory took a long time to be accepted – surgeons did not begin to wear masks in surgery until the 1890s.

Important Discoveries: Louis Pasteur (1860s)

Showed microorganism could be found in air but were not generated by air Demonstrated that there is no microbial contamination when air is withheld Demonstrated that rotting of food is caused by bacteria

Joseph Lister (1860s) Observed that wounds left open to air were prone to infection Concluded that bacteria which Pasteur indicated could rot food might also be cause

of infections Starting using carbolic acid (which he knew was highly poisonous to living

organisms) on patients, his own hands and around hospital rooms Incidence of infection in patients was dramatically decreased – first practice of

antiseptic surgery

Robert Koch (1870s) First person to demonstrate that relationship between a specific microorganism

and a specific disease – this lead to the development of Koch’s postulates

Ignaz Semmelweis:The forgotten scientist Lister has been quoted as saying “Without Semmelweis my

work would be nothing” but who was Semmelweis??? Semmelweis was a Hungarian doctor teaching medicine in

Vienna in the 1840s He observed that childbirth fever (peurperal fever) was

killing around 13% of new mothers in one ward of his hospital which was staffed by doctors and students, while only 2% of new mothers were dying of this same disease in another ward staffed by nurses and midwifes. Why the difference?

In 1947, Semmelweis’ friend Jakob Kolletscha was performing an autopsy on a woman who died of childbirth fever when he punctured his finger with a knife and died of similar symptoms.

Ignaz Semmelweis:The forgotten scientist cont… An autopsy of Kolletscha indicated the same pathological findings as

childbirth fever Semmelweis proposed a relationship between the performing of

autopsies and the spread of childbirth fever He immediately instigated a policy of using chlorinated lime for

washing hands between performing autopsies and examining patients, and the death rate in the ward staffed by doctors and students dropped to a level similar to that of the other ward

In 1848 he extended his policy to washing all instruments that were coming in contact with women in labour and his records indicated that childbirth fever was virtually eliminated from the hospital

A number of different factors limited the speed with which his ideas caught on – when his ideas and results were finally published in 1861 they received much unfavourable criticism

Disgusted with the medical establishment, Semmelweis eventually had a nervous breakdown and was admitted to a mental hospital where he eventually died of infection.

Infectious and contagiousWhat’s the difference?

Contagious diseases are infectious diseases that are capable of spreading rapidly from one person to another by contact or close proximity.

Occurrence of infectious diseaseWithin a population, an infectious disease may be:

Sporadic – occasional occurrence e.g. localised cases of chicken pox

Endemic – regular, continuing occurrence e.g. malaria in parts of Africa

Epidemic – significantly increased occurrence e.g. Horse influenza in NSW 2007

Pandemic – epidemic occurrence in multiple countries e.g. the Spanish flu of 1918-1919

Infectious diseases are caused by pathogens

A pathogen is a parasite, microorganism or other agent which causes an infectious disease.

Infections caused by pathogens Infections caused by pathogens can be

divided into two types: primary and opportunistic.

Primary infections cause disease when they infect a host, they are not usually associated with the host e.g. plague (Yersinia pestis) and influenza.

Opportunistic infections cause disease in some cases, but can be part of normal flora at other times e.g. Pseudomonas, Candida

What is normal flora? Bacteria found normally in or on the body More bacterial than human cells in the body Provide some nutrients (vitamin K) Stimulate immune system, immunity can

be cross-reactive against certain pathogens Prevent colonization by potential

pathogens (antibiotic-associated colitis, Clostridium difficile)

How do pathogens cause disease?

To understand how pathogens cause disease we need to understand how it infects a host and the effects it has on that host.

Three steps in causing disease

1. Transmission of pathogen2. Grow and reproduction of pathogen3. Damage to host

Transmission There must be some way for a successful

pathogen to move from one host to the next.

Modes of transmission: Airborne Waterborne Passed directly from one host to the next by

direct contact or exchange of bodily fluids Present in food Carried by other living organisms (vectors)

Growth and/or Reproduction

In order to cause disease pathogens must be able to grow and/or reproduce in the host.

Damage to the host Damage to the host is generally due to one

or more of the following factors: Invasion of tissue – breakdown of tissue and cell

death. Damage to cells from bacterial enzymes Production of toxins Inhibition of normal cell function Depriving host of nutrients Production of damaging proteins (e.g. viruses

and prions) Body’s own reaction to presence of pathogen:

fever, inflammation and swelling.

Koch’s Postulates Proposed by Robert Koch (1884)as a way of proving

that microorganisms are the cause of a particular disease.

1. Microorganism must be present in every case of the disease

2. Organism must be grown in pure culture from disease hosts

3. Produce the same disease from the pure culture 4. Organism recovered from experimentally infected

hosts

Molecular Postulates Describe virulence factors – more

applicable to new diseases e.g. SARS

1. Virulence gene or its product must be present

2. Virulence gene must transform a non-pathogen into a pathogen

3. Virulence gene must be expressed during disease process

4. Antibodies against gene products are protective

Types of pathogens Pathogens may be cellular or non-cellular. Some parasites may be pathogenic, while

others do not cause diseases directly but are important in transmitting diseases (e.g. fleas transmitting the plague).

Cellular pathogens or pathogenic organisms include: bacteria, protozoa, oomycetes, fungi, worms and arthropods.

Non-cellular pathogens or pathogenic agents include: viruses, viroids and prions.